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Volume contraction in photocured dental resins: The shrinkage-conversion relationship revisited.

Dewaele M, Truffier-Boutry D, Devaux J, Leloup G

Ecole de Médecine Dentaire et de Stomatologie, Université catholique de Louvain, Bruxelles, Belgium; Unité de Chime et de Physique des Hauts Polymères, Université catholique de Louvain, Place Croix du Sud, 1 - Bât. Boltzmann, B-1348 Louvain-la-Neuve Belgium.

Polymerization shrinkage and degree of conversion (DC) of resin composites are closely related manifestations of the same process. Ideal dental composite would show an optimal degree of conversion and minimal polymerization shrinkage. These seem to be antagonistic goals, as increased monomer conversion invariably leads to high polymerization shrinkage values. OBJECTIVES: This paper aims at accurately determining the polymerization volume contraction of experimental neat resins and to link it to the number of actual vinyl double bonds converted in single ones instead of, as generally done, to the degree of conversion. METHODS: Different mixtures of Bis-GMA/TEGDMA (traditionally used monomers) were analyzed. Contraction of the polymers was determined by pycnometry and the use of a density column. DC was determined by the use of Raman spectrometry. RESULTS: An univocal relationship has been found between the volume contraction and the actual number of vinyl double bonds converted into single ones. A contraction value of 20.39cm(3)/mole (of converted C=C) was deduced from 27 measurements. SIGNIFICANCE: This relationship helps in finding solutions to the polymerization shrinkage problem. A reduction of the polymerization shrinkage due to the chemical reaction may obviously be expected from the addition of molecules allowing a decrease in the number of double bonds converted per unit volume of resin matrix, while maintaining the degree of conversion (of Bis-GMA and TEGDMA) and thus the mechanical properties. Further research will be directed at this objective.

Published 13 March 2006 in Dent Mater, 22(4): 359-365.
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